1. Field of the Invention
This invention relates to digital-to-analog converters (DACs). More particularly, this invention relates to current-output DACs employing MOS transistor current sources for the bit currents.
2. Description of the Prior Art
Current-type DACs employing MOS current sources have been used for some time now. Examples of such DACs are shown in U.S. Pat. Nos. 4,558,242 (Tuthill and Minogue) and 4,590,456 (Burton and Real). It is important that the DAC output currents not vary with changing conditions such as power supply voltage, temperature or manufacturing process variations. It is known to achieve good output current stability by designing the circuitry to have a high output impedance, so that the output current closely tracks a reference current which may be made to be very constant.
However, obtaining high output impedance by conventional circuit design requires relatively large IC chip areas, and this results in slower speed of operation and uses more chip "real estate" than desirable. Thus, there is a need for an MOS current-output DAC which can be made small, yet still provide nearly constant output currents in the face of changing conditions.